Windows Internals covering windows server 2008 and windows vista- P14

Windows Internals covering windows server 2008 and windows vista- P14: In this chapter, we’ll introduce the key Microsoft Windows operating system concepts and
terms we’ll be using throughout this book, such as the Windows API, processes, threads, virtual
memory, kernel mode and user mode, objects, handles, security, and the registry.

Internally, each volume shadow copy shown isn’t a complete copy of the drive, so it doesn’t
duplicate the entire contents twice, which would double disk space requirements for every single
copy. Previous Versions uses the copy-on-write mechanism described earlier to create shadow
copies. For example, if the only file that changed between time A and time B, when a volume
shadow copy was taken, is New.txt, the shadow copy will contain only New.txt. This allows VSS
to be used in client scenarios with minimal visible impact on the user, since entire drive contents
are not duplicated and size constraints remain small.
Although shadow copies for previous versions are taken daily (or whenever a Windows Update or
software installation is performed, for example), you can manually request a copy to be taken.
This can be useful if, for example, you’re about to make major changes to the system or have just
copied a set of files you want to save immediately for the purpose of creating a previous version.
You can access these settings by right-clicking Computer on the Start Menu or desktop, selecting
Properties, and then clicking System Protection. You can also open Control Panel, click System
And Maintenance, and then click System. The dialog box shown in Figure 8-27 allows you to
select the volumes on which to enable System Restore (which also affects previous versions) and
to create an immediate restore point and name it.
EXPERIMENT: Mapping Volume Shadow Device Objects
Although you can browse previous versions by using Explorer, this doesn’t give you a permanent
interface through which you can access that view of the drive in an application-independent,
persistent way. You can use the Vssadmin utility (%System-Root%\System32\Vssadmin.exe)
included with Windows to view all the shadow copies taken, and you can then take advantage of
symbolic links to map a copy. This experiment will show you how.
1. List all shadow copies available on the system by using the list shadows command:
1. vssadmin list shadows
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You’ll see output that resembles the following. Each entry is either a previous version copy or a
shared folder with shadow copies enabled.
1. vssadmin 1.1 - Volume Shadow Copy Service administrative command-line tool
2. (C) Copyright 2001-2005 Microsoft Corp.
3. Contents of shadow copy set ID: {dfe617b7-ef2b-4280-9f4e-ddf94c2ccfac}
4. Contained 1 shadow copies at creation time: 8/27/2008 1:59:58 PM
5. Shadow Copy ID: {f455a794-6b0c-49e4-9ae5-e54647fd1f31}
6. Original Volume: (C:)\\?\Volume{f5f9d9c3-7466-11dd-9ba5-806e6f6e6963}\
7. Shadow Copy Volume: \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy1
8. Originating Machine: WIN-SL5V78KD01W
9. Service Machine: WIN-SL5V78KD01W
10. Provider: 'Microsoft Software Shadow Copy provider 1.0'
11. Type: ClientAccessibleWriters
12. Attributes: Persistent, Client-accessible, No auto release,
13. Differential, Auto recovered
14. Contents of shadow copy set ID: {02dad996-e7b0-4d2d-9fb9-7e692be8fe3c}
15. Contained 1 shadow copies at creation time: 8/29/2008 1:51:14 AM
16. Shadow Copy ID: {79c9ee14-ca1f-4e46-b3f0-0dc98f8eb0d4}
17. Original Volume: (C:)\\?\Volume{f5f9d9c3-7466-11dd-9ba5-806e6f6e6963}\
18. Shadow Copy Volume: \\?\GLOBALROOT\Device\HarddiskVolumeShadowCopy2.
19. ...
Note that each shadow copy set ID displayed in this output matches the C$ entries shown by
Explorer in the previous experiment, and the tool also displays the shadow copy volume, which
corresponds to the shadow copy device objects that you can see with WinObj.
2. You can now use the Mklink.exe utility to create a directory symbolic link (for more
information on symbolic links, see Chapter 11), which will let you map a shadow copy into an
actual location. Use the /d flag to create a directory link, and specify a folder on your drive to map
to the given volume device object. Make sure to append the path with a backslash (\) as shown
here:
1. mklink /d c:\old \\?\gLOBaLrOOT\Device\HarddiskVolumeShadowCopy2\
3. Finally, with the Subst.exe utility, you can map the c:\old directory to a real volume using the
command shown here:
1. Subst g: c:\old
You can now access the old contents of your drive from any application by using the c:\old path,
or from any command-prompt utility by using the g:\ path—for example, try dir g: to list the
contents of your drive.
Shadow Copies for Shared Folders
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Windows also takes advantage of Volume Shadow Copy to provide a feature that lets standard
users access backup versions of volumes on file servers so that they can recover old versions of
files and folders that they might have deleted or changed. The feature alleviates the burden on
systems administrators who would otherwise have to load backup media and access previous
versions on behalf of these users.
The Properties dialog box for a volume includes a tab named Shadow Copies, shown in Figure
8-28. An administrator can enable scheduled snapshots of volumes using this tab, as shown in the
following screen. Administrators can also limit the amount of space consumed by snapshots so
that the system deletes old snapshots to honor space constraints.
When a client Windows system (running Windows Vista Business, Enterprise, or Ultimate) maps
a share from a folder on a volume for which snapshots exist, the Previous Versions tab appears in
the Properties dialog box for folders and files on the share, just like for local folders. The Previous
Versions tab shows a list of snapshots that exist on the server, instead of the client, allowing the
user to view or copy a file or folder’s data as it existed in a previous snapshot.
8.6 Conclusion
In this chapter, we’ve reviewed the on-disk organization, components, and operation of Windows
disk storage management. In Chapter 10, we delve into the cache manager, an executive
component integral to the operation of file system drivers that mount the volume types presented
in this chapter. However, next, we’ll take a close look at an integral component of the Windows
kernel: the memory manager.
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9. Memory Management
In this chapter, you’ll learn how Windows implements virtual memory and how it manages the
subset of virtual memory kept in physical memory. We’ll also describe the internal structure and
components that make up the memory manager, including key data structures and algorithms.
Before examining these mechanisms, we’ll review the basic services provided by the memory
manager and key concepts such as reserved memory versus committed memory and shared
memory.
9.1 Introduction to the Memory Manager
By default, the virtual size of a process on 32-bit Windows is 2 GB. If the image is marked
specifically as large address space aware, and the system is booted with a special option
(described later in this chapter), a 32-bit process can grow to be 3 GB on 32-bit Windows and to 4
GB on 64-bit Windows. The process virtual address space size on 64-bit Windows is 7,152 GB on
IA64 systems and 8,192 GB on x64 systems. (This value could be increased in future releases.)
As you saw in Chapter 2 (specifically in Table 2-3), the maximum amount of physical memory
currently supported by Windows ranges from 2 GB to 2,048 GB, depending on which version and
edition of Windows you are running. Because the virtual address space might be larger or smaller
than the physical memory on the machine, the memory manager has two primary tasks:
■ Translating, or mapping, a process’s virtual address space into physical memory so that when a
thread running in the context of that process reads or writes to the virtual address space, the
correct physical address is referenced. (The subset of a process’s virtual address space that is
physically resident is called the working set. Working sets are described in more detail later in this
chapter.)
■ Paging some of the contents of memory to disk when it becomes overcommitted—that is, when
running threads or system code try to use more physical memory than is currently available—and
bringing the contents back into physical memory when needed.
In addition to providing virtual memory management, the memory manager provides a core set of
services on which the various Windows environment subsystems are built. These services include
memory mapped files (internally called section objects), copy-on-write memory, and support for
applications using large, sparse address spaces. In addition, the memory manager provides a way
for a process to allocate and use larger amounts of physical memory than can be mapped into the
process virtual address space (for example, on 32-bit systems with more than 4 GB of physical
memory). This is explained in the section “Address Windowing Extensions” later in this chapter.
Memory Manager Components
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The memory manager is part of the Windows executive and therefore exists in the file
Ntoskrnl.exe. No parts of the memory manager exist in the HAL. The memory manager consists
of the following components:
■ A set of executive system services for allocating, deallocating, and managing virtual memory,
most of which are exposed through the Windows API or kernel-mode device driver interfaces
■ A translation-not-valid and access fault trap handler for resolving hardware-detected memory
management exceptions and making virtual pages resident on behalf of a process
■ Several key components that run in the context of six different kernel-mode system threads:
❏ The working set manager (priority 16), which the balance set manager (a system thread that the
kernel creates) calls once per second as well as when free memory falls below a certain threshold,
drives the overall memory management policies, such as working set trimming, aging, and
modified page writing.
❏ The process/stack swapper (priority 23) performs both process and kernel thread stack
inswapping and outswapping. The balance set manager and the threadscheduling code in the
kernel awaken this thread when an inswap or outswap operation needs to take place.
❏ The modified page writer (priority 17) writes dirty pages on the modified list back to the
appropriate paging files. This thread is awakened when the size of the modified list needs to be
reduced.
❏ The mapped page writer (priority 17) writes dirty pages in mapped files to disk (or remote
storage). It is awakened when the size of the modified list needs to be reduced or if pages for
mapped files have been on the modified list for more than 5 minutes. This second modified page
writer thread is necessary because it can generate page faults that result in requests for free pages.
If there were no free pages and there was only one modified page writer thread, the system could
deadlock waiting for free pages.
❏ The dereference segment thread (priority 18) is responsible for cache reduction as well as for
page file growth and shrinkage. (For example, if there is no virtual address space for paged pool
growth, this thread trims the page cache so that the paged pool used to anchor it can be freed for
reuse.)
❏ The zero page thread (priority 0) zeroes out pages on the free list so that a cache of zero pages
is available to satisfy future demand-zero page faults. (Memory zeroing in some cases is done by a
faster function called MiZeroInParallel. See the note in the section “Page List Dynamics.”)
Each of these components is covered in more detail later in the chapter.
Internal Synchronization
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Like all other components of the Windows executive, the memory manager is fully reentrant and
supports simultaneous execution on multiprocessor systems—that is, it allows two threads to
acquire resources in such a way that they don’t corrupt each other’s data. To accomplish the goal
of being fully reentrant, the memory manager uses several different internal synchronization
mechanisms to control access to its own internal data structures, such as spinlocks.
(Synchronization objects are discussed in Chapter 3.)
Systemwide resources to which the memory manager must synchronize access include the page
frame number (PFN) database (controlled by a spinlock), section objects and the system working
set (controlled by pushlocks), and page file creation (controlled by a guarded mutex). Per-process
memory management data structures that require synchronization include the working set lock
(held while changes are being made to the working set list) and the address space lock (held
whenever the address space is being changed). Both these locks are implemented using pushlocks.
Examining Memory Usage
The Memory and Process performance counter objects provide access to most of the details about
system and process memory utilization. Throughout the chapter, we’ll include references to
specific performance counters that contain information related to the component being described.
We’ve included relevant examples and experiments throughout the chapter.
One word of caution, however: different utilities use varying and sometimes inconsistent or
confusing names when displaying memory information. The following experiment illustrates this
point. (We’ll explain the terms used in this example in subsequent sections.)
EXPERIMENT: Viewing System Memory Information
The Performance tab in the Windows Task Manager, shown in the following screen shot, displays
basic system memory information. This information is a subset of the detailed memory
information available through the performance counters.
The following table shows the meaning of the memory-related values.
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25. PagedPool Commit: 8635 ( 34540 Kb)
26. Driver Commit: 2246 ( 8984 Kb)
27. Committed pages: 73000 ( 292000 Kb)
28. Commit limit: 386472 ( 1545888 Kb)
29. Total Private: 44889 ( 179556 Kb)
30. 0400 svchost.exe 5436 ( 21744 Kb)
31. 0980 explorer.exe 4123 ( 16492 Kb)
32. 0a7c windbg.exe 3713 ( 14852 Kb)
9.2 Services the Memory Manager Provides
The memory manager provides a set of system services to allocate and free virtual memory, share
memory between processes, map files into memory, flush virtual pages to disk, retrieve
information about a range of virtual pages, change the protection of virtual pages, and lock the
virtual pages into memory.
Like other Windows executive services, the memory management services allow their caller to
supply a process handle indicating the particular process whose virtual memory is to be
manipulated. The caller can thus manipulate either its own memory or (with the proper
permissions) the memory of another process. For example, if a process creates a child process, by
default it has the right to manipulate the child process’s virtual memory. Thereafter, the parent
process can allocate, deallocate, read, and write memory on behalf of the child process by calling
virtual memory services and passing a handle to the child process as an argument. This feature is
used by subsystems to manage the memory of their client processes, and it is also key for
implementing debuggers because debuggers must be able to read and write to the memory of the
process being debugged.
Most of these services are exposed through the Windows API. The Windows API has three groups
of functions for managing memory in applications: page granularity virtual memory functions
(Virtualxxx), memory-mapped file functions (CreateFileMapping, CreateFileMappingNuma,
MapViewOfFile, MapViewOfFileEx, and MapViewOfFileExNuma), and heap functions
(Heapxxx and the older interfaces Localxxx and Globalxxx, which internally make use of the
Heapxxx APIs). (We’ll describe the heap manager later in this chapter.)
The memory manager also provides a number of services (such as allocating and deallocating
physical memory and locking pages in physical memory for direct memory access [DMA]
transfers) to other kernel-mode components inside the executive as well as to device drivers.
These functions begin with the prefix Mm. In addition, though not strictly part of the memory
manager, some executive support routines that begin with Ex are used to allocate and deallocate
from the system heaps (paged and nonpaged pool) as well as to manipulate look-aside lists. We’ll
touch on these topics later in this chapter in the section “Kernel-Mode Heaps (System Memory
Pools).”
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Although we’ll be referring to Windows functions and kernel-mode memory management and
memory allocation routines provided for device drivers, we won’t cover the interface and
programming details but rather the internal operations of these functions. Refer to the Windows
Software Development Kit (SDK) and Windows Driver Kit (WDK) documentation on MSDN for
a complete description of the available functions and their interfaces.
9.2.1 Large and Small Pages
The virtual address space is divided into units called pages. That is because the hardware memory
management unit translates virtual to physical addresses at the granularity of a page. Hence, a
page is the smallest unit of protection at the hardware level. (The various page protection options
are described in the section “Protecting Memory” later in the chapter.) There are two page sizes:
small and large. The actual sizes vary based on hardware architecture, and they are listed in Table
9-1.
Note IA64 processors support a variety of dynamically configurable page sizes, from 4 KB up to
256 MB. Windows uses 8 KB and 16 MB for small and large pages, respectively, as a result of
performance tests that confirmed these values as optimal. Additionally, recent x64 processors
support a size of 1 GB for large pages, but Windows does not currently use this feature.
The advantage of large pages is speed of address translation for references to other data within the
large page. This advantage exists because the first reference to any byte within a large page will
cause the hardware’s translation look-aside buffer (or TLB, which is described in the section
“Translation Look-Aside Buffer”) to have in its cache the information necessary to translate
references to any other byte within the large page. If small pages are used, more TLB entries are
needed for the same range of virtual addresses, thus increasing recycling of entries as new virtual
addresses require translation. This, in turn, means having to go back to the page table structures
when references are made to virtual addresses outside the scope of a small page whose translation
has been cached. The TLB is a very small cache, and thus large pages make better use of this
limited resource.
To take advantage of large pages on systems with more than 255 MB of RAM, Windows maps
with large pages the core operating system images (Ntoskrnl.exe and Hal.dll) as well as core
operating system data (such as the initial part of nonpaged pool and the data structures that
describe the state of each physical memory page). Windows also automatically maps I/O space
requests (calls by device drivers to MmMapIoSpace) with large pages if the request is of
satisfactory large page length and alignment. In addition, Windows allows applications to map
their images, private memory, and page-file-backed sections with large pages. (See the
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MEM_LARGE_PAGE flag on the VirtualAlloc, VirtualAllocEx, and VirtualAllocExNuma
functions.) You can also specify other device drivers to be mapped with large pages by adding a
multistring registry value to HKLM\SYSTEM\CurrentControlSet\Control\Session Manager
\Memory Management\LargePageDrivers and specifying the names of the drivers as separately
null- terminated strings.
One side-effect of large pages is that because each large page must be mapped with a single
protection (because hardware memory protection is on a per-page basis), if a large page contains
both read-only code and read/write data, the page must be marked as read/write, which means that
the code will be writable. This means device drivers or other kernel-mode code could, as a result
of a bug, modify what is supposed to be read-only operating system or driver code without
causing a memory access violation. However, if small pages are used to map the kernel, the
read-only portions of Ntoskrnl.exe and Hal.dll will be mapped as readonly pages. Although this
reduces efficiency of address translation, if a device driver (or other kernel-mode code) attempts
to modify a read-only part of the operating system, the system will crash immediately, with the
finger pointing at the offending instruction, as opposed to allowing the corruption to occur and the
system crashing later (in a harder-to-diagnose way) when some other component trips over that
corrupted data. If you suspect you are experiencing kernel code corruptions, enable Driver
Verifier (described later in this chapter), which will disable the use of large pages.
9.2.2 Reserving and Committing Pages
Pages in a process virtual address space are free, reserved, or committed. Applications can first
reserve address space and then commit pages in that address space. Or they can reserve and
commit in the same function call. These services are exposed through the Windows VirtualAlloc,
VirtualAllocEx, and VirtualAllocExNuma functions.
Reserved address space is simply a way for a thread to reserve a range of virtual addresses for
future use. Attempting to access reserved memory results in an access violation because the page
isn’t mapped to any storage that can resolve the reference.
Committed pages are pages that, when accessed, ultimately translate to valid pages in physical
memory. Committed pages are either private and not shareable or mapped to a view of a section
(which might or might not be mapped by other processes). Sections are described in two
upcoming sections, “Shared Memory and Mapped Files” and “Section Objects.”
If the pages are private to the process and have never been accessed before, they are created at the
time of first access as zero-initialized pages (or demand zero). Private committed pages can later
be automatically written to the paging file by the operating system if memory demands dictate.
Committed pages that are private are inaccessible to any other process unless they’re accessed
using cross-process memory functions, such as ReadProcessMemory or WriteProcessMemory. If
committed pages are mapped to a portion of a mapped file, they might need to be brought in from
disk when accessed unless they’ve already been read earlier, either by the process accessing the
page or by another process that had the same file mapped and had previously accessed the page, or
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if they’ve been prefetched by the system. (See the section “Shared Memory and Mapped Files”
later in this chapter.)
Pages are written to disk through normal modified page writing as pages are moved from the
process working set to the modified list and ultimately to disk (or remote storage). (Working sets
and the modified list are explained later in this chapter.) Mapped file pages can also be written
back to disk as a result of an explicit call to FlushViewOfFile or by the mapped page writer as
memory demands dictate.
You can decommit pages and/or release address space with the VirtualFree or VirtualFreeEx
function. The difference between decommittal and release is similar to the difference between
reservation and committal—decommitted memory is still reserved, but released memory is neither
committed nor reserved. (It’s freed.)
Using the two-step process of reserving and committing memory can reduce memory usage by
deferring committing pages until needed but keeping the convenience of virtual contiguity.
Reserving memory is a relatively fast and inexpensive operation under Windows because it
doesn’t consume any committed pages (a precious system resource) or process page file quota (a
limit on the number of committed pages a process can consume—not necessarily page file space).
All that needs to be updated or constructed is the relatively small internal data structures that
represent the state of the process address space. (We’ll explain these data structures, called virtual
address descriptors, or VADs, later in the chapter.)
Reserving and then committing memory is useful for applications that need a potentially large
virtually contiguous memory buffer; rather than committing pages for the entire region, the
address space can be reserved and then committed later when needed. A use of this technique in
the operating system is the user-mode stack for each thread. When a thread is created, a stack is
reserved. (1 MB is the default; you can override this size with the CreateThread and
CreateRemoteThread function calls or change it on an imagewide basis by using the /STACK
linker flag.) By default, the initial page in the stack is committed and the next page is marked as
a guard page (which isn’t committed) that traps references beyond the end of the committed
portion of the stack and expands it.
9.2.3 Locking Memory
In general, it’s better to let the memory manager decide which pages remain in physical memory.
However, there might be special circumstances where it might be necessary for an application or
device driver to lock pages in physical memory. Pages can be locked in memory in two ways:
■ Windows applications can call the VirtualLock function to lock pages in their process working
set. The number of pages a process can lock can’t exceed its minimum working set size minus
eight pages. Therefore, if a process needs to lock more pages, it can increase its working set
minimum with the SetProcessWorkingSetSizeEx function (referred to in the section “Working Set
Management”).
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■ Device drivers can call the kernel-mode functions MmProbeAndLockPages, MmLockPagable-
CodeSection, MmLockPagableDataSection, or MmLockPagableSectionByHandle. Pages locked
using this mechanism remain in memory until explicitly unlocked. No quota is imposed on the
number of pages a driver can lock in memory because (for the last three APIs) the resident
available page charge is obtained when the driver first loads to ensure that it can never cause a
system crash due to overlocking. For the first API, charges must be obtained or the API will return
a failure status.
9.2.4 Allocation Granularity
Windows aligns each region of reserved process address space to begin on an integral boundary
defined by the value of the system allocation granularity, which can be retrieved from the
Windows GetSystemInfo or GetNativeSystemInfo function. This value is 64 KB, a granularity
that is used by the memory manager to efficiently allocate metadata (for example, VADs, bitmaps,
and so on) to support various process operations. In addition, if support were added for future
processors with larger page sizes (for example, up to 64 KB) or virtually indexed caches that
require systemwide physical-to-virtual page alignment, the risk of requiring changes to
applications that made assumptions about allocation alignment would be reduced.
Note Windows kernel-mode code isn’t subject to the same restrictions; it can reserve memory on a
single-page granularity (although this is not exposed to device drivers for the reasons detailed
earlier). This level of granularity is primarily used to pack TEB allocations more densely, and
because this mechanism is internal only, this code can easily be changed if a future platform
requires different values. Also, for the purposes of supporting 16-bit and MS-DOS applications on
x86 systems only, the memory manager provides the MEM_DOS_LIM flag to the
MapViewOfFileEx API, which is used to force the use of single-page granularity.
Finally, when a region of address space is reserved, Windows ensures that the size and base of the
region is a multiple of the system page size, whatever that might be. For example, because x86
systems use 4-KB pages, if you tried to reserve a region of memory 18 KB in size, the actual
amount reserved on an x86 system would be 20 KB. If you specified a base address of 3 KB for an
18-KB region, the actual amount reserved would be 24 KB. Note that the internal memory
manager structure describing the allocation (this structure will be described later) would then also
be rounded to 64-KB alignment/length, thus making the remainder of it inaccessible.
9.2.5 Shared Memory and Mapped Files
As is true with most modern operating systems, Windows provides a mechanism to share memory
among processes and the operating system. Shared memory can be defined as memory that is
visible to more than one process or that is present in more than one process virtual address space.
For example, if two processes use the same DLL, it would make sense to load the referenced code
pages for that DLL into physical memory only once and share those pages between all processes
that map the DLL, as illustrated in Figure 9-1.
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Each process would still maintain its private memory areas in which to store private data, but the
program instructions and unmodified data pages could be shared without harm. As we’ll explain
later, this kind of sharing happens automatically because the code pages in executable images are
mapped as execute-only and writable pages are mapped as copy-on-write. (See the section
“Copy-on-Write” for more information.)
The underlying primitives in the memory manager used to implement shared memory are called
section objects, which are called file mapping objects in the Windows API. The internal structure
and implementation of section objects are described in the section “Section Objects” later in this
chapter.
This fundamental primitive in the memory manager is used to map virtual addresses, whether in
main memory, in the page file, or in some other file that an application wants to access as if it
were in memory. A section can be opened by one process or by many; in other words, section
objects don’t necessarily equate to shared memory.
A section object can be connected to an open file on disk (called a mapped file) or to committed
memory (to provide shared memory). Sections mapped to committed memory are called
pagefilebacked sections because the pages are written to the paging file if memory demands
dictate. (Because Windows can run with no paging file, page-file-backed sections might in fact be
“backed” only by physical memory.) As with any other empty page that is made visible to user
mode (such as private committed pages), shared committed pages are always zero-filled when
they are first accessed to ensure that no sensitive data is ever leaked.
To create a section object, call the Windows CreateFileMapping or CreateFileMappingNuma
function, specifying the file handle to map it to (or INVALID_HANDLE_VALUE for a
page-filebacked section) and optionally a name and security descriptor. If the section has a name,
other processes can open it with OpenFileMapping. Or you can grant access to section objects
through handle inheritance (by specifying that the handle be inheritable when opening or creating
the handle) or handle duplication (by using DuplicateHandle). Device drivers can also manipulate
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section objects with the ZwOpenSection, ZwMapViewOfSection, and ZwUnmapViewOfSection
functions.
A section object can refer to files that are much larger than can fit in the address space of a process.
(If the paging file backs a section object, sufficient space must exist in the paging file and/or RAM
to contain it.) To access a very large section object, a process can map only the portion of the
section object that it requires (called a view of the section) by calling the MapViewOfFile,
MapViewOfFileEx, or MapViewOfFileExNuma function and then specifying the range to map.
Mapping views permits processes to conserve address space because only the views of the section
object needed at the time must be mapped into memory.
Windows applications can use mapped files to conveniently perform I/O to files by simply making
them appear in their address space. User applications aren’t the only consumers of section objects:
the image loader uses section objects to map executable images, DLLs, and device drivers into
memory, and the cache manager uses them to access data in cached files. (For information on how
the cache manager integrates with the memory manager, see Chapter 10.) How shared memory
sections are implemented, both in terms of address translation and the internal data structures, is
explained later in this chapter.
EXPERIMENT: Viewing Memory Mapped Files
You can list the memory mapped files in a process by using Process Explorer from Windows
Sysinternals (www.microsoft.com/technet/sysinternals). To view the memory mapped files by
using Process Explorer, configure the lower pane to show the DLL view. (Click on View, Lower
Pane View, DLLs.) Note that this is more than just a list of DLLs—it represents all memory
mapped files in the process address space. Some of these are DLLs, one is the image file (EXE)
being run, and additional entries might represent memory mapped data files. For example, the
following display from Process Explorer shows a Microsoft Word process that has memory
mapped the Word document being edited into its address space:
You can also search for memory mapped files by clicking on Find, DLL. This can be useful when
trying to determine which process(es) are using a DLL that you are trying to replace.
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9.2.6 Protecting Memory
As explained in Chapter 1, Windows provides memory protection so that no user process can
inadvertently or deliberately corrupt the address space of another process or the operating system
itself. Windows provides this protection in four primary ways.
First, all systemwide data structures and memory pools used by kernel-mode system components
can be accessed only while in kernel mode—user-mode threads can’t access these pages. If they
attempt to do so, the hardware generates a fault, which in turn the memory manager reports to the
thread as an access violation.
Second, each process has a separate, private address space, protected from being accessed by any
thread belonging to another process. The only exceptions are if the process decides to share pages
with other processes or if another process has virtual memory read or write access to the process
object and thus can use the ReadProcessMemory or WriteProcessMemory function. Each time a
thread references an address, the virtual memory hardware, in concert with the memory manager,
intervenes and translates the virtual address into a physical one. By controlling how virtual
addresses are translated, Windows can ensure that threads running in one process don’t
inappropriately access a page belonging to another process.
Third, in addition to the implicit protection virtual-to-physical address translation offers, all
processors supported by Windows provide some form of hardware-controlled memory protection
(such as read/write, read-only, and so on); the exact details of such protection vary according to
the processor. For example, code pages in the address space of a process are marked read-only and
are thus protected from modification by user threads.
Table 9-2 lists the memory protection options defined in the Windows API. (See the VirtualPro-
tect, VirtualProtectEx, VirtualQuery, and VirtualQueryEx functions.)
And finally, shared memory section objects have standard Windows access control lists (ACLs)
that are checked when processes attempt to open them, thus limiting access of shared memory to
those processes with the proper rights. Security also comes into play when a thread creates a
section to contain a mapped file. To create the section, the thread must have at least read access to
the underlying file object or the operation will fail.
Once a thread has successfully opened a handle to a section, its actions are still subject to the
memory manager and the hardware-based page protections described earlier. A thread can change
the page-level protection on virtual pages in a section if the change doesn’t violate the permissions
in the ACL for that section object. For example, the memory manager allows a thread to change
the pages of a read-only section to have copy-on-write access but not to have read/write access.
The copy-on-write access is permitted because it has no effect on other processes sharing the data.
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9.2.7 No Execute Page Protection
No execute page protection (also referred to as data execution prevention, or DEP) causes an
attempt to transfer control to an instruction in a page marked as “no execute” to generate an access
fault. This can prevent certain types of malware from exploiting bugs in the system through the
execution of code placed in a data page such as the stack. DEP can also catch poorly written
programs that don’t correctly set permissions on pages from which they intend to execute code. If
an attempt is made in kernel mode to execute code in a page marked as no execute, the system will
crash with the ATTEMPTED_EXECUTE_OF_NOEXECUTE_MEMORY bugcheck code. (See
Chapter 14 for an explanation of these codes.) If this occurs in user mode, a
STATUS_ACCESS_VIOLATION (0xc0000005) exception is delivered to the thread attempting
the illegal reference. If a process allocates memory that needs to be executable, it must explicitly
mark such pages by specifying the PAGE_EXECUTE, PAGE_EXECUTE_READ, PAGE_
EXECUTE_READWRITE, or PAGE_EXECUTE_WRITECOPY flags on the page granularity
memory allocation functions.
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On 32-bit x86 systems, the flag in the page table entry to mark a page as nonexecutable is
available only when the processor is running in Physical Address Extension (PAE) mode. (See the
section “Physical Address Extension (PAE)” later in this chapter.) Thus, support for hardware
DEP on 32-bit systems requires loading the PAE kernel (\%SystemRoot%\System32
\Ntkrnlpa.exe), even if that system does not require extended physical addressing (for example,
physical addresses greater than 4 GB). The operating system loader does this automatically unless
explicitly configured not to by setting the BCD option pae to ForceDisable.
On 64-bit versions of Windows, execution protection is always applied to all 64-bit processes and
device drivers and can be disabled only by setting the nx BCD option to AlwaysOff. Execution
protection for 32-bit programs depends on system configuration settings, described shortly. On
64-bit Windows, execution protection is applied to thread stacks (both user and kernel mode),
user-mode pages not specifically marked as executable, kernel paged pool, and kernel session pool
(for a description of kernel memory pools, see the section “Kernel-Mode Heaps (System Memory
Pools).” However, on 32-bit Windows, execution protection is applied only to thread stacks and
user-mode pages, not to paged pool and session pool.
The application of execution protection for 32-bit processes depends on the value of the BCD nx
option. The settings can be changed by going to the Data Execution Prevention tab under
Computer, Properties, Advanced System Settings, Performance Settings. (See Figure 9-2.) When
you configure no execute protection in the Performance Options dialog box, the BCD nx option is
set to the appropriate value. Table 9-3 lists the variations of the values and how they correspond to
the DEP settings tab. Thirty-two-bit applications that are excluded from execution protection are
listed as registry values under the key HKLM\SOFTWARE\Microsoft\Windows NT\Current-
Version\AppCompatFlags\Layers, with the value name being the full path of the executable and
the data set to “DisableNXShowUI”.
On Windows Vista (both 64-bit and 32-bit versions) execution protection for 32-bit processes is
configured by default to apply only to core Windows operating system executables (the nx BCD
option is set to OptIn) so as not to break 32-bit applications that might rely on being able to
execute code in pages not specifically marked as executable, such as self-extracting or packed
applications. On Windows Server 2008 systems, execution protection for 32-bit applications is
configured by default to apply to all 32-bit programs (the nx BCD option is set to OptOut).
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Note To obtain a complete list of which programs are protected, install the Windows Application
Compatibility Toolkit (downloadable from www.microsoft.com) and run the Compatibility
Administrator Tool. Click System Database, Applications, and then Windows Components. The
pane at the right shows the list of protected executables.
Even if you force DEP to be enabled, there are still other methods through which applications can
disable DEP or their own images. For example, regardless of the execution protection options that
are enabled, the image loader (see Chapter 3 for more information about the image loader) will
verify the signature of the executable against known copy-protection mechanisms (such as
SafeDisc and SecuROM) and disable execution protection to provide compatibility with older
copy-protected software such as computer games.
Additionally, to provide compatibility with older versions of the Active Template Library (ATL)
framework (version 7.1 or earlier), the Windows kernel provides an ATL thunk emulation
environment. This environment detects ATL thunk code sequences that have caused the DEP
exception and emulates the expected operation. Application developers can request that ATL
thunk emulation not be applied by using the latest Microsoft C++ compiler and specifying the
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/NXCOMPAT flag (which sets the IMAGE_DLLCHARACTERISTICS_NX_COMPAT flag in
the PE header), which tells the system that the executable fully supports DEP. Note that ATL
thunk emulation is permanently disabled if the AlwaysOn value is set.
Finally, if the system is in OptIn or OptOut mode and executing a 32-bit process, the
SetProcessDEPPolicy function allows a process to dynamically disable DEP or to permanently
enable it. (Once enabled through this API, DEP cannot be disabled programmatically for the
lifetime of the process.) This function can also be used to dynamically disable ATL thunk
emulation in case the image wasn’t compiled with the /NXCOMPAT flag. On 64-bit processes or
systems booted with AlwaysOff or AlwaysOn, the function always returns a failure. The
GetProcessDEPPolicy function returns the 32-bit per-process DEP policy (it fails on 64-bit
systems, where the policy is always the same—enabled), while the GetSystemDEPPolicy can be
used to return a value corresponding to the policies in Table 9-3.
EXPERIMENT: looking at DEP Protection on Processes
Process Explorer can show you the current DEP status for all the processes on your system,
including whether the process is opted-in or benefiting from permanent protection. To look at the
DEP status for processes, right-click any column in the process tree, choose Select Columns, and
then select DEP Status on the Process Image tab. Three values are possible:
■ DEP (permanent) This means that the process has DEP enabled because it is a “necessary
Windows program or service.”
■ DEP This means that the process opted-in to DEP, either as part of a systemwide policy to
opt-in all 32-bit processes or because of an API call such as SetProcessDEPPolicy.
■ Nothing If the column displays no information for this process, DEP is disabled, either because
of a systemwide policy or an explicit API call or shim.
The following Process Explorer window shows an example of a system on which DEP is enabled
for all programs and services.
Software Data Execution Prevention
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For older processors that do not support hardware no execute protection, Windows supports
limited software data execution prevention (DEP). One aspect of software DEP reduces exploits of
the exception handling mechanism in Windows. (See Chapter 3 for a description of structured
exception handling.) If the program’s image files are built with safe structured exception handling
(a feature in the Microsoft Visual C++ compiler that is enabled with the /SAFESEH flag), before
an exception is dispatched, the system verifies that the exception handler is registered in the
function table (built by the compiler) located within the image file. If the program’s image files
are not built with safe structured exception handling, software DEP ensures that before an
exception is dispatched, the exception handler is located within a memory region marked as
executable.
Two other methods for software DEP that the system implements are stack cookies and pointer
encoding. The first relies on the compiler to insert special code at the beginning and end of each
potentially exploitable function. The code saves a special numerical value (the cookie) on the
stack on entry and validates the cookie’s value before returning to the caller saved on the stack
(which would have now been corrupted to point to a piece of malicious code). If the cookie value
is mismatched, the application is terminated and not allowed to continue executing. The cookie
value is computed for each boot when executing the first user-mode thread, and it is saved in the
KUSER_SHARED_DATA structure. The image loader reads this value and initializes it when a
process starts executing in user mode. (See Chapter 3 for more information on the shared data
section and the image loader.)
The cookie value that is calculated is also saved for use with the EncodeSystemPointer and
DecodeSystemPointer APIs, which implement pointer encoding. When an application or a DLL
has static pointers that are dynamically called, it runs the risk of having malicious code overwrite
the pointer values with code that the malware controls. By encoding all pointers with the cookie
value and then decoding them, when malicious code sets a nonencoded pointer, the application
will still attempt to decode the pointer, resulting in a corrupted value and causing the program to
crash. The EncodePointer and DecodePointer APIs provide similar protection but with a
per-process cookie (created on demand) instead of a per-system cookie.
Note The system cookie is a combination of the system time at generation, the stack value of the
saved system time, the number of page faults, and the current interrupt time.
9.2.8 Copy-on-Write
Copy-on-write page protection is an optimization the memory manager uses to conserve physical
memory. When a process maps a copy-on-write view of a section object that contains read/write
pages, instead of making a process private copy at the time the view is mapped, the memory
manager defers making a copy of the pages until the page is written to. For example, as shown in
Figure 9-3, two processes are sharing three pages, each marked copy-on-write, but neither of the
two processes has attempted to modify any data on the pages.
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